Articulations (Joints)

Introduction to Joints

Joints, or articulations, are the sites where two or more bones meet. They are the weakest parts of the skeleton but are essential for providing mobility and holding the skeleton together.

• Articulation: The location at which bones connect.

• Functions of joints:

  • Provide mobility to the skeleton

  • Hold the skeleton together

Classification of Joints

Structural Classification

Structural classification is based on the material binding the bones and the presence or absence of a joint cavity. The three main types are:

• Fibrous joints – Bones joined by fibrous tissue; no joint cavity; mostly immovable.

• Cartilaginous joints – Bones united by cartilage; no joint cavity; variable mobility.

• Synovial joints – Bones separated by a fluid-filled joint cavity; freely movable.

Functional Classification

Functional classification is based on the degree of movement allowed:

• Synarthroses – Immovable joints

• Amphiarthroses – Slightly movable joints

• Diarthroses – Freely movable joints

Fibrous Joints

General Features

Fibrous joints are connected by dense connective tissue and lack a joint cavity. Most are immovable.

• Sutures: Found between skull bones; interlocking junctions filled with connective tissue fibers. Allow growth during youth; fuse in adulthood (synostoses).

• Syndesmoses: Bones connected by ligaments; movement varies from immovable to slightly movable (e.g., tibia-fibula, radius-ulna).

• Gomphoses: Peg-in-socket joints (e.g., tooth in alveolar socket); connected by the periodontal ligament.

Cartilaginous Joints

General Features

Cartilaginous joints unite bones with cartilage and lack a joint cavity. Two main types:

• Synchondroses: Bones united by hyaline cartilage; all are immovable (synarthrotic). Examples: epiphyseal plates, first rib-sternum joint.

• Symphyses: Articulating surfaces covered with hyaline cartilage fused to a pad of fibrocartilage; amphiarthrotic (slightly movable). Examples: intervertebral joints, pubic symphysis.

Synovial Joints

General Structure

Synovial joints are characterized by a fluid-filled joint cavity and are all freely movable (diarthroses). They are the most common type of joint in the body.

• Articular cartilage: Hyaline cartilage covering bone ends, reducing friction.

• Joint (synovial) cavity: Space containing synovial fluid.

• Articular capsule: Double-layered capsule enclosing the joint cavity (outer fibrous layer and inner synovial membrane).

• Synovial fluid: Lubricates and nourishes articular cartilage.

• Reinforcing ligaments: Strengthen and support the joint.

Friction-Reducing Structures

• Bursae: Flattened sacs lined with synovial membrane, containing synovial fluid; reduce friction where structures rub together.

• Tendon sheath: Elongated bursa that wraps around a tendon, especially in the digits.

Stability of Synovial Joints

• Articular surfaces: Shape determines possible movements.

• Ligaments: Unite bones and prevent excessive movement.

• Muscle tendons: Cross joints and help stabilize them.

Range of Motion

• Nonaxial: Slipping movements only

• Uniaxial: Movement in one plane

• Biaxial: Movement in two planes

• Multiaxial: Movement in or around all three planes

Types of Synovial Joints

Plane Joints

Articular surfaces are essentially flat, allowing only slipping or gliding movements. These are the only examples of nonaxial joints. Example: intercarpal joints.

Hinge Joints

Cylindrical projection of one bone fits into a trough-shaped surface of another. Motion is along a single plane (uniaxial), permitting flexion and extension only. Examples: elbow, interphalangeal joints.

Pivot Joints

Rounded end of one bone protrudes into a ring formed by another bone and ligament. Only uniaxial movement is allowed. Examples: joint between the atlas and dens (atlantoaxial joint), proximal radioulnar joint.

Condyloid (Ellipsoidal) Joints

Oval articular surface of one bone fits into a complementary depression in another. Biaxial joints permit all angular motions. Examples: radiocarpal (wrist) joints, metacarpophalangeal (knuckle) joints.

Saddle Joints

Similar to condyloid joints but allow greater movement. Each articular surface has both concave and convex areas. Example: carpometacarpal joint of the thumb.

Ball-and-Socket Joints

Spherical head of one bone articulates with a cuplike socket of another. Multiaxial joints permit the most freely moving synovial joints. Examples: shoulder and hip joints.

Movements at Synovial Joints

Gliding Movements

One flat bone surface glides or slips over another. Examples: intercarpal and intertarsal joints, articular processes of vertebrae.

Angular Movements

• Flexion: Decreases the angle of the joint.

• Extension: Increases the angle of the joint.

• Dorsiflexion and plantar flexion: Up and down movement of the foot at the ankle.

• Abduction: Movement away from the midline.

• Adduction: Movement toward the midline.

• Circumduction: Movement describes a cone in space (combination of flexion, abduction, extension, and adduction).

Rotation

Turning of a bone around its own long axis. Examples: between first two vertebrae, hip and shoulder joints.

Special Movements

• Supination and pronation: Rotational movement of the forearm; supination turns the palm up, pronation turns it down.

• Inversion and eversion: Movements of the sole of the foot inward (inversion) or outward (eversion).

• Protraction and retraction: Anterior and posterior movement in a transverse plane (e.g., mandible).

• Elevation and depression: Lifting a body part superiorly or moving it inferiorly (e.g., mandible).

• Opposition: Movement of the thumb to touch the tips of other fingers.

Summary Table: Types of Joints and Movements

Additional info: The formal name for the joint between the atlas and the dens is the atlantoaxial joint. The joint between the occiput and the atlas is the atlanto-occipital joint.